The present invention relates to a medical device for use in the dental field. Such medical devices, in particular dental implants, are preferably made of metals such as titanium and titanium alloy. In the hydroxylated and hydrophilic state such materials can provide an excellent osseointegration .

In the area of the soft tissue, by contrast, such materials are associated with disadvantages. First, the darkish metal implant shows through the soft tissue, which may negatively influence the visual impression. Second, in the case of gingival recession, bare metal may become visible, the so called black triangle, which is highly unwanted . In order to overcome said disadvantages a large number of documents disclose metal implants with a ceramic sleeve arranged in the area of the soft tissue contact surface (for example DE 298 20 971, US 5,152,687, GB 2 139 095). Ceramic sleeves of this kind are, for example, adhesively bonded onto the implant. As a result, however, a number of problems arise which have not as yet been resolved. Using a ceramic sleeve as a separate structural part almost unavoidably results in a micro-gap, which can cause bacterial contamination. Alternatively such a sleeve can be fitted by sintering. However, high temperatures have to be used for the sintering process, which can cause a partial oxidation of the metallic body of the implant. This has a disadvantageous effect on osseointegration. Moreover, different coefficients of thermal expansion of the materials involved can, particularly during cooling, result in the development of microstress, which can lead to hairline fractures and cracks.

Coatings with silicate glass and with silicate-glass- modified ceramics are also known, for example from document WO 01/74730. A disadvantage of these is, once again, the possibility of microstress occurring as a result of the firing process at high temperatures and the at least partial oxidation of the titanium surface. Finally, the color of all said coatings and sleeves cannot be influenced to the desired extent, resulting in a negative visual impression.

US 6 019 600 discloses an autoclavable, abrasion resistant dental mirror comprising a holder and a mirror element mounted in the holder. The mirror element comprises a sapphire body, an optically reflective aluminum layer on the rear surface of the sapphire body, and a protective layer on the reflective layer. Also DE 200 13 791 discloses a highly reflective dental mirror. The problem of the invention is therefore to provide a medical device which combines the excellent mechanical properties of the metals, especially of titanium, with the good esthetic properties of highly reflecting coatings.

In particular, the problem of the present invention is to provide a medical device for the restoration, replacement or correction of teeth which combines the excellent mechanical properties of the metals, especially of titanium, with the good esthetic properties of highly reflecting coatings. This problem is solved with a medical device according to claim 1 and a method for preparing such a medical device according to claim 14. Further preferred embodiments are defined by the dependent claims. In particular, the problem is solved with a medical device for use in the dental field made of titanium, zirconium, niobium, hafnium, tantalum, vanadium, aluminum, steel or alloys thereof, wherein the surface of said medical device is at least partially coated with a highly reflecting interlayer and said interlayer is coated with a protective layer, characterized in that the interlayer is a metal film selected from the group of silver, aluminum, rhodium, gold, palladium, platinum, titanium, zirconium as well as alloys and mixtures thereof, and the protective layer comprises a transparent ceramic material selected from the group of silica (Si0 ₂ ) , silicon nitride (Si _x N _y , wherein x is from 1 to 4 and y is from 1 to 8), alumina (AI2O3) , aluminum nitride (A1N) , zirconia (Zr0 ₂ ), titania (Ti0 ₂ ) , and mixtures thereof. The medical device according to the present invention is selected from the group of dental implants, orthodontic appliances, crowns, bridges and abutments. Most preferably, the medical device is a dental implant which may be a one-part dental implant or a two-part dental implant.

The medical device according to the present invention is for use in the dental field, and in particular, for use in the visible front part of the mouth. The medical device is made of titanium, zirconium, niobium, hafnium, tantalum, vanadium, aluminum, steel or alloys thereof. Titanium or a titanium alloy is a preferred material. In case of titanium alloys in particular alloys are preferred in which zirconium, niobium, tantalum, vanadium and/or hafnium have been added. A most preferred alloy is a titanium/zirconium alloy; an example of which is Roxolid ^® . Titanium and titanium alloys are biocompatible and have excellent corrosion resistance in body fluids.

The surface of said medical device is at least partially coated with a highly reflecting interlayer and said interlayer is continuously coated with a protective layer. The interlayer is a metal film preferably selected from the group of silver, aluminum, rhodium, gold, palladium, platinum as well as alloys and mixtures thereof.

The protective layer comprises a transparent ceramic material selected from the group of silica (Si0 ₂ ) , silicon nitride (Si _x N _y , wherein x is from 1 to 4 and y is from 1 to 8, preferably S13N4, alumina (AI2O3) , aluminum nitride (A1N) , zirconia (Zr0 ₂ ) , titania (Ti0 ₂ ) and mixtures thereof. Within the context of the present invention the expression transparent ceramic stands for a ceramic having an absorption coefficient k of the complex index of refraction which is 0 or very small. Therefore, only a small part of the light gets absorbed in the material. By tuning the retardation R=2nd (n: refractive index, d: film thickness) of the protective layer, for example by the thickness of the protective layer, the desired color can be achieved. I.e., depending on the retardation of the protective layer, the visual appearance of the medical device will be for example pink, yellow or whitish. Furthermore, it protects the reflecting interlayer from scratches, which could negatively influence the optical appearance . The medical device according to the present invention has excellent esthetic characteristics, since it provides a color that resembles teeth or soft tissue due to the presence of the highly reflecting interlayer and the protective layer. The mechanical integrity is very good because, for example, the strain at cracking is higher than 0.5% which corresponds to the elastic limit of titanium, i.e. the interlayer and the protective layer do not crack upon elastic deformations of the medical device. In addition, the interlayer shows a strong adhesion to the surface of the medical device, i.e., there is no spalling upon elastic deformations of the medical device. Finally, such a medical device has a good integration with both, soft and hard tissue. In a preferred embodiment of the present invention the highly reflecting interlayer is a silver film. A high reflectivity of the reflecting interlayer normally leads to a good esthetic result in the mouth. Further, the silver film can be produced using standardized production methods which result in low cost.

In another preferred embodiment of the present invention the highly reflecting interlayer is an aluminum film. It can be produced using standardized production methods which result in low cost. In another preferred embodiment of the present invention the highly reflecting interlayer is an rhodium film.

In another embodiment of the present invention an adhesive layer is arranged between the surface of the medical device and the highly reflecting interlayer in order to improve the adhesion of the highly reflecting interlayer onto the surface of the medical device. The presence of an adhesive layer improves the long-term stability of the medical device. Preferably, the material of the adhesion layer is selected from the group of titanium, zirconium, silver, aluminum and rhodium or a mixture thereof. An adhesive layer made of silver, aluminum or rhodium better matches the crystal structure of the interlayer, if said interlayer partly comprises silver, aluminum or rhodium, which results in a better adhesion. The adhesion layer has typically a thickness of less than 15 nm, preferably between 5 and 10 nm, including 5, 6, 7, 8, 9 and 10 nm, most preferably 10 nm.

In another embodiment of the present invention the ceramic material of the protective layer comprises zirconia or consists of zirconia. Zirconia is well tolerated by patients, is well accepted on the market and can therefore help to prevent allergic reactions which could be caused by the material of the highly reflecting interlayer. Furthermore, such a protective layer has a high fracture toughness . In a further embodiment of the present invention the ceramic material of the protective layer is stabilized with yttria (Y ₂ 0 ₃ ) , preferably ytteria-stabilized zirconia. Such a protective layer provides excellent hardness and fracture toughness and can therefore effectively protect the highly reflecting interlayer from damages such as scratches and corrosion.

In a further embodiment, the ceramic material of the protective layer comprises titania (Ti0 ₂ ) or consists of titania (Ti0 ₂ ) . Titania is well tolerated by patients and can therefore help to prevent allergic reactions which could be caused by the material of the highly reflecting interlayer .

In another embodiment of the present invention the highly reflecting interlayer has a thickness from 30 to 520 nm preferably from 45 to 55 nm, most preferably 50 nm. In addition to mechanical strength and stability, the highly reflecting interlayer is less susceptible to fracture because it has a thickness in the nanometer range, as opposed to a micrometer or even millimeter range. In a further embodiment the retardation R of the protective layer is from 400 nm to 1000 nm, preferably from 600 nm to 850 nm. A retardation of 725 nm to 850 nm results in a color of the peri-implant mucosa which is not distinguishable from normal mucosa, whereas a retardation of 450 nm to 600 nm results in a white color, and a retardation of 600 to 725 nm in a yellow color.

The retardation corresponds to the optical path difference between two light rays reflected at the air-protecting layer interface and reflected at the protective layer- reflecting interlayer interface , i.e. one is reflected at the upper interface (directed away from the medical device) and the other one at the lower interface (directed to the medical device) . If for example the desired color of the device is pink, destructive interference has to occur for greenish light (wavelength λ=530 nm) as the remaining blue and red light will combine to pink. In this case, the optical thickness for second order interference (nd=3X/4) has to be 397.5 nm (n stands for the refractive index and d for the thickness) . This means that the incoming light gets reflected at the interlayer after it has passed a virtual distance of three quarters of its wavelength. The retardation (R=3A/2=2nd) is 795 nm and destructive interference occurs as the two reflected light rays are now completely out of phase. A retardation of about 450 to 600 nm results in a whitish color of the medical device, whereas a retardation of about 600 to 725 nm results in a yellow color and a retardation of about 725 to 850 nm results in a pink color of the medical device .

In another embodiment of the present invention the coated surface of the final medical device, that is the surface which is coated with the highly reflecting interlayer and the protective layer, has an L*-value higher than 80, preferably from 85 to 95, most preferably 90. The L*-value is one dimension of the L*a*b*-system (L*a*b* color space) . Said system has three dimensions, that is L* for lightness and a* and b* for the color-opponent dimensions. For the purpose of this invention L* is the most important value, since L* determines the lightness of the coating system, while a* and b* only determine the hue of the color. For standard metals such as titanium, the values for a* and for b* are around 0 as they do not have any color. The best optical appearance of the medical device could be obtained in cases in which the coated surface of the medical device has an L*-value from 85 to 90. The expression one-part implant stands for a single component implant, in which anchoring part and abutment part are configured in one piece.

The expression two-part implant stands for an anchoring part of a two-part implant system (two component implant system) . In addition to the anchoring part a two-part implant system comprises a separate abutment. The extent of the surface of the medical device which is coated with the highly reflecting interlayer and the protective layer is dependent on the nature of the medical device. For example for orthodontic appliances and bridges at least 50%, preferably more than 75% and most preferably more than 90% of the surface is covered by the highly reflecting interlayer and the protective layer. In a most preferred embodiment the surface is completely covered by the highly reflecting interlayer and the protective layer. In contrast thereto, for dental implants, abutments and crowns, the surface is preferably only partly covered by the highly reflecting interface and the protective layer.

Generally, an abutment comprises an apical portion configured to engage the dental implant, a transition portion bordering coronally on the portion and on which coronally borders a coronal portion for attachment of the crown. The term "coronal" is here and throughout this application used to indicate a direction towards a head end or trailing end of the component discussed. Likewise, the term "apical" indicates _, a direction towards an insertion end of the component. Thus, apical and coronal are opposite directions. Furthermore, the term "axial direction" is used throughout this application to indicate a direction taken from the coronal end to the apical end, or vice versa.

In a preferred embodiment of the present invention the transition portion of the abutment is covered with the highly reflecting interlayer and the protective layer. In another embodiment of the present invention both, the transition portion and the coronal portion of the abutment, are covered by the highly reflecting interlayer and the protecting layer, whereas the apical portion configured to engage the dental implant is free from these layers allowing a very precise connection to the implant.

In another embodiment of the present invention the medical device is a two-part dental implant for attachment of artificial teeth. Said implant is generally in a cylindrical or conical shape, having an apical end with a body portion and a coronal end with a neck portion intended to receive an abutment, and a transition portion being arranged in axial direction between the body portion and the neck portion. The- neck portion includes an unthreaded part which is in coronal direction outwardly tapering and ends in a shoulder portion with an inwardly- tapering surface. Preferably, the total length of the implant is 6 - 19 mm in the axial direction. The body portion with the apical end is preferably 50 to 90% of the total length of the implant in axial direction. The neck portion with the coronal end which is essentially be intended to be in contact with the soft tissue covers preferably 10 to 40% of the total length of the implant in axial direction. The transition portion covers preferably 0 to 40% of the total length of the implant in axial direction. The neck portion has preferably a length of 1 to 4, most preferably 1.5 to 3 mm.

Said neck portion is at least partially covered with the highly reflecting interlayer and the protecting layer. In a preferred embodiment the shoulder portion of the neck portion is not covered by the highly reflecting interlayer and the protecting layer. Preferably in apical direction, at least 50%, most preferably at least 75% of the circumferential surface area of the transition portion is covered with said highly reflecting interlayer and the protective layer. Most preferably, with the exception of the shoulder portion, the neck portion, that is unthreaded part which is in coronal direction outwardly tapering, is completely covered with the highly reflecting interlayer and the protecting layer. Thanks to said coated surface almost no color change or a brightening can be observed in the peri-implant mucosa.

In a further embodiment of the present invention not only the neck portion but also the transition portion of the implant ^" is at least partly coated with said interlayer and said protective layer. Preferably, at least 25%, most preferably at least 50% of the coronal circumferential surface area of the transition portion is covered with said interlayer and said protective layer.

In another embodiment of the present invention the transition portion and the neck portion of such a two part implant comprise the highly reflecting interlayer and the protecting layer, but the highly reflecting interlayer and the protecting layer of the transition portion exhibits a different color than the highly reflecting interlayer and the protecting layer of the neck portion.

In another embodiment of the present invention both, the transition portion and the neck portion with the exception of the shoulder portion of such a two-part implant, are completely covered with the highly reflecting interlayer and the protecting layer, whereas the body portion is bare metal, which is preferably surface treated, to ensure an optimal osseointegration . In another embodiment of the present invention both, the transition portion and the neck portion of such a two-part implant, are completely covered with the highly reflecting interlayer and the protective layer, whereas the body portion is bare metal, which is preferably surface treated, to ensure an optimal osseointegration .

According to another embodiment of the present invention, the edge of the interlayer and the protective layer facing toward the body portion and extending along the circumference of the implant is formed in a curved shape, with at least one rise, preferably two such rises, and one dip, preferably two such dips, so that the natural shape of the bone is simulated. In this way it can be ensured that, ideally, all the soft tissue contact surfaces are provided with the interlayer and the protective layer. With such a configuration of the edge of the interlayer and the protective layer, it is possible to effectively avoid the color of the basic material of the implant showing through the soft tissue in the implanted state. The configuration of the edge can in this case be made universal to match most bone shapes but can also be made individual, that is to say for each individual implant. In particular, it is possible, by means of CAD/CAM, to exactly register the surface of the implant to be coated via the bone shape of the patient and to provide the implant with the interlayer and the protective layer accordingly .

The present invention also provides a method for producing a medical device as described above. In a first step the surface to be coated is treated by sputter deposition (for example magnetron sputtering, ion assisted sputtering, gas flow sputtering, radio frequency sputtering) , electron beam physical vapor deposition or evaporation to obtain the interlayer, preferably by direct current magnetron sputtering. In a second step said interlayer obtained in the first step is coated with a ceramic protection layer by sputter deposition (for example magnetron sputtering, ion assisted sputtering, gas flow sputtering, radio frequency sputtering) , chemical vapor deposition, atomic layer deposition, pulsed laser deposition or by anodization of a metallic layer deposited, on top of the highly reflecting interlayer, preferably by reactive direct current magnetron sputtering.

According to a further preferred embodiment the surface of the medical device which has to be coated is mechanically (for example by milling, turning, grinding, polishing or blasting) pretreated, or laser pretreated, or chemically (for example by etching or electropolishing) pretreated, or ion pretreated (ion bombardment) , or pretreated by a combination thereof. The pretreatment can be carried out together with the surface treatment of other parts of the implant, for example when roughening the surface of the body portion or directly before carrying out the coating process . The pretreated surface has preferably a surface roughness S _a from 50 to 1000 nm, most preferably from 80 to 600 nm.

In addition, if the coating is carried out directly under vacuum any oxide layer, which may be present, is removed by such a pretreatment which allows for a better adhesion of the reflector coating. In the present invention the surface roughness S _a (mean surface roughness) is defined according to ISO 25178. It may be measured for example by a confocal microscope explorer (NanoFocus AG) (scan mode: piezo, lens: 20x, light source: LED with green light, measuring range: 798 μπι x 798 μπι) . Preferably, the following operators are used for the determination of the roughness using the software Soft Analysis XT Version 5.1.1.5944 (NanoFocus AG): Non- measured points filled (Replaces non-measured points by a smooth shape calculated from the neighbors. Dilates the non-measured points by 1.57 μπι) ; Spatial Filtering (Median (denoising) filter, filter size 3x3); Levelling (Least square plane. Levelling by subtraction); Form Removal (Polynomial of order 2. Apply non-measured points to the form alone); Thresholding (Material ratio 0.1% - 99.9%. Reduce height/depth; without the need of a mathematical filter, thresholding artificially truncates an image at 0.1% from the top and 99.9% from the bottom at the same time) ; Filtering -> Roughness (Gaussian Filter, cut-off 31 μπι, cut surface edges) .

Figures

Fig 1 shows a schematic view of the medical device ;

Fig. 2a to Fig show three specific embodiments of the present invention; Fig. 3 shows a further embodiment of the present invention;

Figures 4a to show spectral reflectance curves of

Ti, Zr0 ₂ films on Ti and on 50 nm thick interlayers of Al or Ag;

Figure 5 shows the color change of Ti samples;

Figures 6a to 6c now color change of Ti-Zr0 ₂

Figures 7a to 7c show the color change of Ti-Al-Zr0 ₂ samples ;

Figures 8a to 8c show the color change of Ti-Ag-Zr0 ₂ samples ;

Fig. 1 shows a schematic view of the medical device according to the present invention. The base body 5 of the medical device 1 is made of titanium, zirconium, niobium, hafnium, tantalum, vanadium, aluminum, steel or alloys thereof. The surface of said medical device is at least partially coated with a highly reflecting interlayer 10 and said interlayer 10 is coated with a protective layer 15. The interlayer 10 is a metal film selected from the group of silver, aluminum, rhodium, gold, palladium, platinum, titanium, zirconium as well as alloys and mixtures thereof. The protective layer 15 comprises a transparent ceramic material selected from the group of silica (Si0 ₂ ) , silicon nitride (Si _x N _y , wherein x is from 1 to 4 and y is from 1 to 8, preferably Si ₃ N ₄ ) ) , alumina (AI2O3) , aluminum nitride (A1N) , zirconia (Zr0 ₂ ), titania (Ti0 ₂ ) , and mixtures thereof.

FIG. 2a shows a two-part implant 1', that is the anchoring part of a two part implant system, which is an example for a medical device according to the present invention. Such a two-part implant 1' is made of titanium, zirconium, niobium, hafnium, tantalum, vanadium, aluminum, steel or alloys thereof, preferably of titanium or of a titanium alloy. Said implant 1' is in a cylindrical or conical shape, having an apical end 25 with a body portion 20 and a coronal end 35 with a neck portion 30 intended to receive an abutment, and a transition portion 40 being arranged in axial direction A between the body portion 20 and the neck portion 30. The neck portion 30 includes an unthreaded part 31 which is in coronal direction outwardly tapering and ends in a shoulder portion 32 with an inwardly-tapering surface. The body portion 20 is intended to be directed against the bone tissue in the implanted state, whereas the neck portion 30 is intended to be directed against the soft tissue in the implanted state. The transition portion 40 may be directed towards the soft tissue and the bone tissue depending on the depth to which the implant is screwed or on the tissue reaction. The neck portion 30 is at least partially covered with the highly reflecting interlayer 10 and the protecting layer 15.

Fig. 2B shows another embodiment of the present invention. In contrast to the embodiment of Fig. 2A not only the neck implant is at least partly coated with said interlayer and said protective layer. Preferably, at least 25%, most preferably at least 50% of the coronal circumferential surface area of the transition portion is covered with said interlayer and said protective layer.

Fig. 2C shows another embodiment of the present invention. In contrast to the embodiment of Fig. 2A with the exception of the shoulder portion 32 the neck portion 30 and transition portion 40 of the implant is completely coated with said interlayer and said protective layer. The body portion 20 is bare metal, which is preferably surface treated, to ensure optimal osseointegration.

Fig. 3 shows another embodiment of an implant 1' . In this case, the edges 45 of the highly reflecting layer 10 and the protecting layer 15 are formed along the circumference of the implant 1' in a curved shape, with at least one rise 50 and one dip 55, so that, in the implanted state, the natural shape of the bone is simulated by these edges 45.

Example

All films are deposited on a 0.5 mm thick Ti grade 4 sheet (ThyssenKrupp Materials Schweiz AG, il, Switzerland) . All samples are punched to a diameter of 15 mm. The samples are differently pretreated. Pickled, but else untreated samples will be referred to as rolled. Other surface treatments include polishing (0.02 μπι Si0 ₂ + H ₂ 0 ₂ ) , fine- blasting with glass beads (40 to 70 μιη Si0 ₂ , 4 bar) and sandblasting (Biloxit No. 360, 4 bar). Deposition (PVD Products sputter system, Wilmington, Massachusetts, USA) :

- Zr0 ₂ : 300 Watt, 50 seem (cm ³ /min) Ar flow, 4.75 seem O2 flow, 5 mTorr chamber pressure, 12 rpm substrate rotation, deposition at room temperature (without substrate heating), approx. 40 nm/min

- Ag: 250 Watt, 10 seem Ar flow, 5 mTorr chamber pressure, 12 rpm substrate rotation, deposition at room temperature, approx. 60 nm/min

- Al : 200 Watt, 10 seem Ar f-low, 5 mTorr chamber pressure, 12 rpm substrate rotation, deposition at room temperature, approx. 12 nm/min

The optical evaluation of the samples was performed by spectrophotometric measurements using a spectrophotometer CM 2600-d (Konica Minolta, Tokyo, Japan) . The resulting reflectance curves and L*a*b*-values were examined with the manufacturer's software SpectraMagic (Konica Minolta). The observer angle was set to 2° and the D65 standard light source with 100% UV with specular reflection included was used. For the measurement of diffused illumination the device uses an integrating sphere with apertures of 3 mm in diameter (SAV - small area view) . Before each measurement the spectrophotometer was zero- calibrated with a black box (CM-A 32) and then calibrated with a white plate (CM-A 142) . The mean reflectance and L*a*b*-values were automatically averaged from three measurements of each specimen. The following samples were tested:

Figures 4a to 4d show spectral reflectance curves of Ti, Zr0 ₂ films on Ti and on 50 nm thick interlayers of Al or Ag. Fig 4a: uncoated (sample 1: white standard, sample 2: polished substrate, sample 3: rolled Ti, sample 4: glass- blasted Ti, sample 5: sandblasted Ti); Fig. 4b: pink (sample 1: gum shade, sample 2: rolled Ti, sample 3: rolled Al, sample 4: rolled Ag) ; Fig. 4c: yellow (sample 1: A3 shade, sample 2: rolled Ti, sample 3: rolled Al, sample 4: rolled Ag) and Fig. 4d: white (sample 1: white standard, sample 2: rolled Ti, sample 3: rolled Al, sample 4: rolled Ag) . The used standards are a gum shade for the pink films, an A3 tooth shade for the yellow films and the spectrophotometer calibration standard CM-A 142 for the white films. A3 tooth shade is a tooth shade on the VITA Classical Shade Guide used by dentists.

Zr0 ₂ films on bare Ti are too dark for an application in the dental area. The lightness is the highest for samples with a 50 nm thick interlayer of Ag, even though Ag is not completely opaque at this thickness, since approximately 2% of the light can be transmitted through a 50 nm thick layer of Ag. The lightness of the coating system with Al as interlayer material is also higher than Ti and, especially, than the gum and A3 tooth shade. The samples with improved lightness are promising for esthetical coatings .

Pig maxillae test The influence of several parameters, such as color, surface treatment of the substrate, coating system and mucosa thickness, on the esthetic appearance was investigated. For statistical reasons, six pig maxillae were used. A palatal mucosa flap, which exhibits similarities to human mucosa with respect to color and texture, was prepared for each maxilla according to the procedure described in Jung et al, in vitro color changes of soft tissue caused by restorative materials, in International Journal of Periodontics & Restorative Dentistry, 2007, 27(3): p. 251-257). Tissue grafts with a thickness of 1 mm and 2 mm were prepared, which were placed beneath the mucosal flap. This flap exhibited a thickness of 1 mm, corresponding to the critical case in human patients with grayish appearance of prosthetic appliances through soft tissue. Spectrophotometric measurements were taken from the region where samples were placed underneath the flap. The differences AL*, Aa* and Ab* were then calculated by subtracting the baseline measurement (tissue flap on bone) from the measurement of the sample below soft tissue. The overall color change ΔΕ was calculated by the following equation:

ΔΕ = [ (AL* ) ² + (Aa*) ² + (Ab*) ² ] ⁰ ' ⁵

In literature, a threshold value of ΔΕ > 3.7 is considered to be a clinically distinguishable color difference in the intraoral environment. Figure 5 shows the color change of Ti samples:

- sample 1: rolled (left bar: mucosal flap 1 mm, central bar mucosal flap 2 mm, right bar mucosal flap 3 mm) ;

- sample 2: polished (left bar: mucosal flap 1 mm, central bar mucosal flap 2 mm, right bar mucosal flap

3 mm) ;

- sample 3: glass-blasted (left bar: mucosal flap 1 mm, central bar mucosal flap 2 mm, right bar mucosal flap 3 mm) ;

- sample 4: sandblasted (left bar: mucosal flap 1 mm, central bar mucosal flap 2 mm, right bar mucosal flap 3 mm) ;

- sample 5: machined (left bar: mucosal flap 1 mm, central bar mucosal flap 2 mm, right bar mucosal flap 3 mm) .

Figures 6a to 6c show the color change of Ti-Zr0 ₂ samples. The thickness of the mucosal flap is in figure 6a 1 mm, in figure 6b 2 mm and in figure 6c 3 mm.

- Sample 1: rolled (left bar: pink, central bar: yellow, right bar: white); - sample 2: polished (left bar: pink, central bar: yellow, right bar: white);

- sample 3: glass-blasted (left bar: pink, central bar: yellow, right bar: white);

- sample 4: sandblasted (left bar: pink, central bar: yellow, right bar: white);

- sample 5: machined (left bar: pink, central bar: yellow, right bar: white) .

Figures 7a to 7c show the color change of Ti-Al-Zr0 ₂ samples. The thickness of the mucosal flap is in figure 7a 1 mm, in figure 7b 2 mm and in figure 7c 3 mm.

- Sample 1: rolled (left bar: pink, central bar: yellow, right bar: white);

- sample 2: polished (left bar: pink, central bar: yellow, right bar: white);

- sample 3: glass-blasted (left bar: pink, central bar: yellow, right bar: white);

- sample 4: sandblasted (left bar: pink, central bar: yellow, right bar: white) ;

- sample 5: machined (left bar: pink, central bar: yellow, right bar: white) .

Figures 8a to 8c show the color change of Ti-Ag-Zr0 ₂ samples. The thickness of the mucosal flap is in figure 8a 1 mm, in figure 8b 2 mm and in figure 8c 3 mm.

- Sample 1: rolled (left bar: pink, central bar: yellow, right bar: white);

- sample 2: polished (left bar: pink, central bar: yellow, right bar: white);

- sample 3: glass-blasted (left bar: pink, central bar: yellow, right bar: white) ; - sample 4: sandblasted (left bar: pink, central bar: yellow, right bar: white);

- sample 5: machined (left bar: pink, central bar: yellow, right bar: white) .

All tested samples revealed an overall color change in covering mucosa. The color change decreases with increasing tissue thickness, as it can be observed in the bar chart for Ti-Zr0 ₂ , Ti-Al-Zr0 ₂ and Ti-Ag-Zr0 ₂ samples (figures 6a to 6c, 7a to 7c and 8a to 8c) . The only critical case (mucosa thickness of 1 mm) is regarded for further discussion. Dark or gray appearance of the covering mucosa was pronounced for all coatings with sandblasted substrates. Furthermore, Ti and Ti-Zr0 ₂ samples would not be suitable for the proposed applications since all ΔΕ values lay well above the critical threshold value of 3.7 and they strongly decrease the lightness of the mucosal flap. Slight deviations around ΔΕ of 3.7 can be observed for Ti-Al-Zr0 ₂ samples. Ti-Ag-Zr0 ₂ samples even revealed a brightening of the soft tissue (ΔΕ > 3.7 with positive AL* values).